E. coli B contains enzymes which can be assigned as mucopeptidhydrolase. They degrade cell wall mucopeptides of the same organism specifically into smaller molecules and are therefore valuable tools for structural analysis of these compounds. According to their specificity the five mucopeptidehydrolysases described below are defined as D-alanincarboxypeptidase. mucoendopeptidase. muramylamidase, N-acetylglucosaminidase and E. coli lysozyme. Their possible role as autolysins in the living cell is discussed.